In machine tools, it is known to measure a part using a contact probe by moving a contact probe along a path such that the probe contacts a surface of the part at a number of points on the surface. A location of the points of contact can be determined from signals from the machine tool and contact probe and an appropriate shape fitted to these points to obtain a representation of the surface. It is also known to rotate the part with the contact probe fixed in position. Examples of these methods are described in U.S. Pat. No. 3,866,829.
In U.S. Pat. No. 3,866,829, the contact probe is movable parallel to the rotational axis to take measurements at different heights on the part. However, it is also known to move the contact probe in a direction perpendicular to the rotational axis such that inner and outer circles of a part can be measured, for example as disclosed in EP0744678.
Errors in the measurement can result in inaccuracies in the generated representation. In particular, in circumstances when the part is rotated and the contact probe is stationary, a diameter of a cylindrical/circular part cannot be measured directly, but the measured position has to be related to a centre of the part in order to determine a diameter. The rotation can provide information on the runout of the part about the centre of rotation, but a position of the centre of rotation cannot be determined directly nor the off-centre error of the part with respect to the centre of rotation. It is also not possible to separate the 1st order circularity of the part from the axis centre drift or the off-centre error of the part.
U.S. Pat. No. 4,852,402 discloses a measurement method wherein a contact probe is moved simultaneously with rotation of a gear. The contact probe is moved in a direction that is orthogonal to a movement direction of a point on a surface of the gear tooth in contact with the contact probe. Deviations from an ideal involute shape of the tooth are recorded.
U.S. Pat. No. 6,154,713 discloses a method of determining a path of a contact probe when measuring a part that is simultaneously rotated with movement of the contact probe such that measurement of a surface always occurs in a preferred measurement direction, normal to the surface.
U.S. Pat. No. 6,327,788 discloses a method of measuring a set of radial values of a cylindrical part at a specified height with a nominally straight contact probe by rotating the part relative to the probe when the probe is located at a first, fixed radial position to obtain a first set of radial values and then moving the probe to a second, fixed radial position diametrically opposed to the first position and rotating the part to obtain a second set of radial values. Sets of values may be obtained at each radial position at a plurality of different heights. In this way, for each point on the part, two measurements of radius are obtained. A mean of the measurements of radius at each point is obtained to compensate for errors in the straightness of the contact probe.
In an alternative embodiment, rather than averaging measurements of radius at each point, each set of radial values is fitted to a circle using a least squares algorithm. An average of the fitted circles is obtained. The fitted circle for one of the sets of radial values is then subtracted from the average circle and the result added to the raw radial values of that set.
FIGS. 1a to 1c schematically illustrate this method for a single height wherein a first circle with radius R1 is fitted to radial values obtained for a first position of the contact probe and a second circle with radius R2 is fitted to radial values obtained for a second position of the contact probe. An average radius, AV, is obtained for values R1 and R2. A difference between the average radius AV and a radius for one of the sets of data, in the illustration R1, is then added to the radial values to which the circle with radius R1 was fitted to obtain a data set that represents the final shape.
It is disclosed that this second embodiment reduces angular phase errors which may occur if the angular separation of the two positions of the contact probe are not accurately known (i.e. the “same point” measured for the two positions of the contact probe may be, in fact, slightly different points because of errors in the angular measurement at each position. Therefore, simple averaging of the radial position for each point may be false averaging because the measurements are for slightly different points).
Such a method may cancel out errors resulting from the lack of straightness of the contact probe but errors from other sources, such as the part being off-centre from the axis of rotation, deformation of other parts of the measurement system or runout, may still produce significant inaccuracies in the measurement of the part.